This invention relates an all-electronic system for the detection, recognition and positive identification of a particular repetitive or non-repetitive sound patterns and more particularly to an apparatus for the selective monitoring and recognition of emergency signals, such as sirens to remote control traffic signal devices.
Briefly, the present invention has primary application to detection of emergency signals and accomplishes its stated function by means of precise frequency discrimination circuits, sequence detection circuits, timed gating circuits, noise rejection circuits, comparator and preamplifier circuits, and repetition counting circuits. Consecutive tones of different frequency must occur to enable delay timers that emit a trigger pulse to a counter chain to actuate a traffic signal relay. Frequency discrimination is accomplished by band pass filters or by IC phase locked loop tone decoders. The various circuits can be readjusted to recognize almost any kind of predetermined repetitive sound pattern while retaining the ability to reject all other unwanted sounds.
The particular application and embodiments described are designed to detect and recognize the sound of a particular operating mode of an emergency vehicle siren known as a "yelp", for the purpose of controlling the traffic signals at an intersection making it easier and safer for the emergency vehicle to traverse the intersection. The system is capable of rejecting all extraneous sounds and sound combinations including other siren operating modes known as "wail" and "high-low". The system also is capable of North/South and East/West directional discrimination. The purpose of making the system responsive to the "yelp" operating mode is because that mode is normally used by emergency vehicle operators when they approach traffic intersections and, therefore, would entail little or no modification to the normal siren usage pattern. Should an emergency vehicle operator, for some reason, wish to make no change in the traffic signal cycle of an intersection he is approaching, he has the option of using any siren mode other than the "yelp".
By way of further explanation, the audio characteristic of the "yelp" operating mode consists of a continuously changing audio tone that begins at a frequency as low as 500 Hz and sweeps to a frequency as high as 1600 Hz and then sweeps back down again to the low frequency, this constituting a single sweep cycle. The sweep cycle is then repeated at a rate of one to four cycles per second. The exact frequency range covered and the exact sweep cycle repetiton rate depends on the particular model and type of siren. The circuits of the present invention accomodate and recognize the full range of "yelp" frequencies and repetition rates mentioned above.
The utility of a system whereby the traffic signals at an intersection are remotely controlled by the driver of an approaching emergency vehicle is thoroughly explained in U.S. Pat. No. 3,550,078, which discloses a system utilizing a photovoltaic detector at the traffic signal and a special high-intensity lamp mounted on each vehicle.
The prior art includes a number of systems having the capability of responding to particular sounds such as sirens or automobile horns. Representative systems are described in U.S. Pat. Nos. 3,568,144 and 3,735,342, both of which are designed to be mounted in a vehicle for the purpose of alerting the driver of the nearby presence of an emergency vehicle siren and, in one case, also the presence of an automobile horn and a train whistle. Neither of these patents make any mention of traffic signal control.
A system responsive to a predetermined pattern of sounds for controlling a traffic signal light is disclosed in U.S. Pat. No. 3,992,656. No directional discrimination capability, however, exists in the system disclosed in this patent.
Before reviewing the above patents in further detail, it is necessary to clarify the distinction between (1) the capability to respond to an audio tone or a predetermined sequence of tones with little or no ability to discriminate against unwanted audio signals that happen to contain the same tone or tone sequence (a tone decoder) and (2) the capability to detect and recognize a particular sound pattern along with the ability to reject all unwanted sounds and sound combinations (a sound pattern discriminator). The former (1) is typified, for example, by a telephone touch-tone system which establishes an artificial, controlled environment in which all the tones and tone sequences that can occur are known. A tone decoder, for instance, that is designed to respond to a predetermined tone sequence characterizing a seven-digit local telephone number would not respond to the first seven digits of any ten-digit, long distance number because the first seven digits of all ten-digit numbers never duplicate any seven-digit number. By the same token, any spurious signals that could cause false responses are adequately filtered or attenuated before reaching the tone decoder. Thus, in a controlled electrical environment, there is little need for the tone decoder to have any special means for rejecting unwanted signals because such signals are adequately attenuated beforehand or, by design, are not permitted to occur.
The latter, (2) is typified, for example, by a busy traffic intersection, which is a natural, uncontrolled environment in which a wide variety of unpredictable sounds and sound combinations may occur. A sound pattern discriminator, for instance, that is designed to detect and recognize the sound of an emergency vehicle siren, must be able to discriminate against and reject such sounds as engine exhaust noise, transmission gear whine, electric horns on automobiles, air horns on trucks, the screeching of brakes, the squealing of tires, and the ever-present, broad-band wind noise. Any circuit that is limited in its ability to reject such extraneous sounds, although it may be useful as as tone decoder in a controlled environment, has little practical value in an uncontrolled environment where it would generate a high percentage of false responses. It also is highly desirable for a system to be able to determine the direction from which the vehicle siren sound is coming for optimum control of the traffic light at an intersection.
Refer now to U.S. Pat. No. 3,568,144, which describes an apparatus, the preferred embodiment of which is claimed to be capable of responding to the sound of a train whistle, an automobile horn, and an emergency vehicle siren and display each response separately. It accomplishes this aim by means of three channels, the circuitry of each including a bandpass filter; one filter being tuned to the characteristic frequency of train whistles, the second being tuned to the characteristic frequency of automobile horns, and the third being tuned to the characteristic frequency of sirens.
The above described systems are not totally effective for two important reasons. First, the use of one bandpass filter to respond to the characteristic frequency of automobile horns does not work because automobile horns do not have a single characteristic frequency. The frequency of a horn varies with the make and model of automobile. Moreover, most automobiles carry two horns, one of low pitch and one of high pitch, to produce a more pleasing tone. If the pass band of the filter were made so broad so as to include the characteristic frequencies of most horns, the system would have no discriminating ability and would respond to most other sounds. Exactly the same reasoning holds true for a train whistle. Although the frequency range for various train whistles is narrower than various horns, the frequency range for whistles overlaps the frequency range for horns. Obviously, a siren does not have a single characteristic frequency, but sweeps a rather wide spectrum, as explained in a previous paragraph, which fully overlaps the frequency ranges of both horns and whistles. The second reason is that, even with narrow-band filters, the circuit has very poor discriminating ability. Most street noises have a complex spectrum that contains many audio components of different frequencies and these noises would cause almost constant false triggering, rendering the circuit useless.
Refer now to U.S. Pat. No. 3,735,342 which relates to a tone-responsive circuit capable of responding to the sound of an emergency vehicle siren. The system of this patent is an improvement over the previous circuits in that sounds of three different frequencies must be detected within a predetermined time period, ten seconds, by means of three bandpass filters before a response is obtained. An SCR sequencing circuit is used so the sounds must occur in a predetermined sequence. There is no delay time built into the sequencer except for the inherent turn-on time of an SCR which is typically less than 0.5 microsecond. Since the period of one cycle of a 1000 Hz tone is 1 millisecond, from a practical standpoint in audio work, a period as short as 0.5 microsecond may be considered to be instantaneous. Thus, three simultaneous tones at the proper frequencies will cause the circuit to respond, as will the same three tones occurring in any sequence whatever, so long as there is at least a 1 to 2 microsecond overlap. The system of this patent does not include any effective means of rejecting unwanted sounds and, therefore, can be easily triggered by any broad-band noise source. At best, this circuit may be considered to be a tone detector for a three-tone signal, but it would be ineffective as a useful sound pattern discriminator.
The system of U.S. Pat. No. 3,992,656 overcomes some, but not all, of the disadvantages of the above prior art systems. The '656 system detects siren frequencies in a sequential or reverse sequential, order to control a traffic signal light. This system, however, does not have the capability of directional discrimination, nor does it respond uniquely to a composite ascending/descending sequence of frequencies.